1. Field of the Invention
The invention relates to an exhaust gas purification device for an engine.
2. Description of the Related Art
As a catalyst for reducing and purifying nitrogen oxides (NOx) included in an exhaust gas discharged from the engine, a catalyst is known which absorbs or stores the NOx included in the exhaust gas to carry it therein when the air-fuel ratio of the exhaust gas flowing thereinto is larger (leaner) than the stoichiometric air-fuel ratio, and reduces and purifies the NOx carried therein when the air-fuel ratio of the exhaust gas flowing thereinto becomes a stoichiometric air-fuel ratio or smaller than the stoichiometric air-fuel ratio. An engine provided with the above-mentioned catalyst (hereinafter referred to as—NOx catalyst—) is disclosed in Unexamined Japanese Patent Publication No. 2004-68690.
The engine disclosed in the Publication No. 2004-68690 comprises six cylinders, which is divided into two cylinder groups. Each cylinder group is connected to an exhaust branch pipe. Further, the exhaust branch pipes are connected to a common exhaust pipe at their downstream ends. A NOx catalyst is positioned in the common exhaust pipe.
The exhaust gas also includes sulfur oxides (SOx) in addition to NOx. Therefore, the NOx catalyst can also carry SOx in addition to the NOx. When the NOx catalyst carries SOx, i.e. is contaminated by the sulfate, the capacity of the NOx catalyst to carry NOx is decreased. Therefore, in order to maintain the high capacity of the NOx catalyst to carry NOx, SOx should be removed from the NOx catalyst. In this connection, SOx can be removed from the NOx catalyst, i.e. the contamination of the NOx catalyst by the sulfate is regenerated, when the temperature of the NOx catalyst is increased to a temperature at which SOx can be removed from the NOx catalyst and the exhaust gas having the stoichiometric or rich (in particular, slightly rich) air-fuel ratio is supplied to the NOx catalyst.
According to the engine disclosed in the Publication No. 2004-68690, in order to remove SOx from the NOx catalyst, a following process for regenerating the sulfate contamination of the NOx catalyst is performed. That is, the air-fuel ratio of the exhaust gas discharged from one of the cylinder groups is controlled to a rich air-fuel ratio, while the air-fuel ratio of the exhaust gas discharged from other cylinder groups is controlled to a lean air-fuel ratio. Then, the exhaust gas having a rich air-fuel ratio (hereinafter referred to as—rich exhaust gas—) and the exhaust gas having a lean air-fuel ratio (hereinafter referred to as—lean exhaust gas—) are mixed with each other and flow into the NOx catalyst. In this case, a rich degree of the rich exhaust gas and a lean degree of the lean exhaust gas are controlled such that the air-fuel ratio of the exhaust gas resulting from the mixture of the rich exhaust gas and the lean exhaust gas becomes the stoichiometric air-fuel ratio.
In this case, the air-fuel ratio of the exhaust gas flowing into the NOx catalyst is controlled to the stoichiometric air-fuel ratio. In addition, when the rich exhaust gas and the lean exhaust gas are mixed with each other, the hydrocarbon (HC) included in the rich exhaust gas reacts with the oxygen included in the lean exhaust gas. Therefore, the heat produced by the reaction of the HC and the oxygen increases the temperature of the exhaust gas and thus the temperature of the NOx catalyst. Thereby, the temperature of the NOx catalyst is increased to the temperature at which the SOx can be removed from the NOx catalyst and the exhaust gas having a stoichiometric air-fuel ratio is supplied to the NOx catalyst. As a result, SOx is removed from the NOx catalyst.
An engine is known which comprises a charcoal canister for adsorbing and storing fuel vapor produced in an fuel tank. In this engine, in order to prevent that an activated charcoal of the canister is filled with the fuel vapor, when the engine is operated, the fuel vapor is discharged into an intake pipe from the canister.
The fuel vapor discharged into the intake pipe is introduced into the cylinders. In the engine disclosed in the Publication No. 2004-68690, if the fuel vapor is discharged from the canister into the intake pipe when the sulfate contamination regeneration process is performed, the amount of the fuel supplied into each cylinder is increased by the amount of the discharged fuel vapor. In this case, in particular, the amount of the fuel in the cylinder, from which the rich exhaust gas is discharged when the sulfate contamination regeneration process is performed, becomes excessively large. Therefore, the fuel may not burn in the cylinder.
The object of the invention is to ensure that the fuel burns in the cylinder in which the mixture gas is smaller (richer) than the stoichiometric air-fuel ratio when the process for regenerating the sulfate contamination of the NOx catalyst is performed.